This invention relates to the laying of concrete slabs, including sidewalks, driveways, and the like, and has particular reference to the placement and support of elongated strips used in such operations, prior to pouring of the concrete. The invention is principally concerned with so-called control joint strips, which control the cracking of the concrete as it sets and cures, but also is applicable to the placement of other components, such as headers, that are used in connection with the laying of concrete.
In a typical process of laying a concrete slab, a suitable bed or base is graded to receive the concrete, and forms are provided to define the area upon which the concrete slab is to be laid, to confine the concrete to this area, and to serve as guides for leveling the surface of the concrete. Because concrete contracts as it sets and cures, a frequent practice is to provide means for controlling the resulting cracking of the concrete, and to promote cracking along regular lines or planes rather than in a haphazard and unsightly fashion. Such control means sometimes are referred to as expansion joints, but are more correctly called control joints.
One practice has been to saw cut grooves or score lines in the surface of the concrete as soon as is practical after it is poured, thereby weakening the slab along the score lines so that cracking hopefully will occur beneath them. In addition to being a time-consuming and expensive operation, saw-cutting has the disadvantage that cracking can commence during the delay between pouring and cutting, necessitated by the setting of the concrete.
As an alterative to saw cutting, elongated divider strips have been placed in the concrete, at or near the surface, to serve the same weakening function. One way to position such strips is to nail them, or otherwise attach them, to the forms on opposite sides of the slab, if spacing permits. Another approach, for larger slabs, is to mount the strips on the upper ends of stakes that are set in the ground in rows and carefully driven to the proper level to hold the strips very close to the surface of the concrete. Another approach has been to set plastic strips in the wet concrete after it has been poured. A strip of this type, with a tear-off top strip facilitating placement, is shown in U.S. Pat. No. 3,352,217.
While all of these approaches are satisfactory, in varying degrees, each has characteristics that make its results less than the optimum obtainable. Forms often are not close enough together to permit strips to be nailed to them, and the need for detachment when the forms are removed is a complication. Moreover, concrete settles as it sets, and rigidly set strips are not free to settle with the concrete.
When strips are mounted on the upper ends of stakes, the stakes must be very carefully set and leveled, and these strips also are not free to settle with the concrete as it settles. Of course, the stakes also are lost because they must remain in the slab.
Tear-top plastic strips have been popular because of their relative inexpensiveness and ease of installation, and also because the strips remain free to settle with the concrete. These strips, however, cannot be set prior to the pouring of the concrete, and, in fact, can be placed only after at least rough finishing has been completed. Thus, they cannot serve as guides in the finishing or "screeding" operation, a useful function performed by rigidly supported control joint elements.
A principal objective of the present invention is to provide an improved system of setting control joint strips and the like which has the advantageous features of the prior system, but without the significant disadvantages thereof.